A secondary battery is a battery capable of being charged and discharged by using an electrode material having excellent reversibility, and one of most popular commercialized examples is a lithium secondary battery. The lithium secondary battery may be used not only as a small power source for small IT devices, such as a smart phone, a portable computer, and an electronic paper, but is also expected to be applied as a medium/large power source mounted on a means of transportation, such as an automobile, or used in a power storage of a power supply network, such as a smart grid.
When a lithium metal is used as a negative electrode material of a lithium secondary battery, short-circuit of the battery may occur due to formation of a dendrite or there is a risk of explosion. Therefore, instead of the lithium metal, crystalline carbon, such as graphite and artificial graphite, soft carbon, hard carbon, and carbon-based active materials to which lithium may be intercalated and deintercalated, are widely used. However, as application of secondary battery expands, there is a demand for higher capacity and higher output of a secondary battery. Accordingly, non-carbon negative electrode materials, which exhibits a capacity of 500 mAh/g or higher and may be alloyed with lithium, (e.g., silicon (Si), tin (Sn), or aluminum (Al)) are spotlighted as replacements of carbon-based negative electrode materials.
From among the above-stated non-carbon negative electrode materials, silicon has the largest theoretical capacity of about 4,200 mAh/g, and thus it is very important to commercialization of silicon in terms of capacity. However, since the volume of silicon increases by four times during charging, an electrical connection between active materials and an electrical connection between a current collector and the active material may be destroyed due to the volume change during charging and discharging, and an irreversible reaction, such as formation of a solid electrolyte interface (SEI) layer (e.g., Li2O) due to corrosion of the active material based on an electrolyte, may occur. As a result, service life of silicon-based negative electrode is deteriorated, and the life deterioration is the major obstacle for commercialization.
While many methods have been proposed in order to achieve a relatively high capacity cell with minimized volume expansion and contraction of an active material to improve its service life, the most likely method for commercialization is to make silicon particles to nano particles. However, although the silicon particles have nano size, even nano-sized silicon particles were not sufficient for commercialization because of the refinement of particles due to repeated volume expansion/shrinkage and a rapid deterioration of service life due to the same. Therefore, for commercialization of silicon-based materials, it is required to design particles capable of maximizing the capacity of a cell while suppressing volume change during charging/discharging, and a manufacturing technique thereof is demanded.